1. Field of the Invention
The present invention relates to lithographic methods and, more particularly to lithographic methods for characterizing and monitoring lithographic exposure tool imaging performance.
2. Description of Related Art
Imaging performance, with respect to a lithographic exposure tool, is generally understood to describe an exposure tool's ability to accurately produce an image of an object. The evaluation of this imaging performance is generally performed first to fully characterize performance over the tool's exposure field and second to monitor tool performance, as part of a process control scheme, when the tool is used to produce images for the manufacture of, for example, an integrated circuit (IC).
For this first task, test pattern reticles or masks have been developed by lithographic tool manufacturers and users that have a number of specially designed test structures placed at a number of locations within the exposure field. An example of such a test pattern having nine groupings of test structures spread over an exposure field is seen, for example, in FIG. 14 of U.S. Pat. No. 4,908,656, issued Mar. 13, 1990 to Suwa et al., which is of different test structures formed therein are designed to enable the evaluation of the different factors that influence imaging performance.
Generally, the evaluation of imaging performance begins by exposing the test pattern at different locations on a substrate using a matrix of exposure conditions, for example as indicated in the exposure time vs. focus matrix depicted in FIG. 17 of Suwa et al. Once this matrix is completed, the various test structures are evaluated for each test pattern. While a variety of criteria are evaluated to fully characterize imaging performance, the measurement of the size of a test structure, referred to as its critical dimension (CD), is among the most important. As these test images are generally quite small, a CD of 0.25 micron (.mu.m) or less is typical for today's high density ICs, a scanning electron microscope designed for such critical dimension measurements (CD-SEM) is generally used. In this manner, the tool's imaging performance is characterized and a best set of exposure conditions selected.
Once the exposure conditions are selected and the lithographic tool begins to produce images in a manufacturing mode, imaging performance must still be evaluated to ensure that it remains within process control limits. Typically, users monitor imaging performance by measuring the CD of a test structure, developed for this second task, that is placed within the exposure field. In this manner, the test structure is present in each exposure field on the substrate or wafer and can be measured. As known, the number of measurements per wafer and the number of wafers measured per lot, or group of wafers, will vary as a function of the process control scheme employed. As mentioned for characterizing imaging performance, these in-process measurements are also generally performed using a CD-SEM.
However, as the size of the features of integrated circuits continues to shrink, the use of CD-SEMs for measuring critical dimensions becomes problematic for several reasons. First, as feature size becomes smaller, small changes in the size across an exposure field become more critical. For example, while a variation in size of .+-.0.01 micron (.mu.m) is only 1% of a 1 .mu.m feature, it is 10% of a 0.1 .mu.m feature. As a result, adequately evaluating imaging performance across an exposure field requires that more sites within each field be measured to allow for more critical adjustments to the tool. Thus the 9 sites shown in FIG. 14 of Suwa et al., are more often 15 or 18 sites now. Thus even with a state of the art CD-SEM, the 49 exposure fields in FIG. 17 of Suwa et al. at 18 sites per field adds considerable additional time to an evaluation. As this additional time can require additional measurement tools and as a state of the art CD-SEM is expensive, the purchase of any additional measurement tools can significantly impact manufacturing costs. A third problem is the ability of even state of the art CD-SEMs to resolve the test structures adequately to make accurate measurements as these structure's sizes approach the resolution capability of the CD-SEM. A fourth problem, contamination caused by the electron beam, generally only affects the use of a CD-SEM for in-process measurements. Thus the very nature of bombarding a structure with an electron beam and the resultant secondary emissions that are captured for imaging, can also contaminate devices in the adjacent ICs. As a result, wafers used for in-process measurements as part of a process control scheme, are sometimes discarded or the photoresist layer removed, the wafer cleaned and reprocessed. Either alternative resulting in increased costs and reduced yields. Thus, alternate methods for characterization of imaging performance and in-process monitoring of that performance are needed.
One such alternate method reported to provide improved accuracy over CD-SEM measurements employs an electrical measurement of an array of test structures. The test structures of this method are formed in a conductive layer overlying a special test substrate where the structures have attached contact regions. Thus, a resist layer is exposed and the pattern of test structures developed and etched. After removing the resist layer, the conductivity of the etched features are measured by an electrical means. Using such parameters as the specific electroconductivity of the conductive layer and the length of the etched feature, a linewidth is calculated. This methodology has gained some acceptance characterizing a tool's imaging performance, especially for small CDs. However, as the procedure involves a special substrate having a conductive layer and several processing steps after formation of an image to pattern the conductive layer, it is not suitable for in-process monitoring of imaging performance, and thus is limited in its use to tool characterization. As a result, an alternate method must be employed for in-process monitoring, thus requiring an additional step of determining the relationship between the electrical measurements and in-process monitoring measurements.
Thus it would be desirable to have a method of forming a test structure and method for using that test structure for both full characterization of imaging performance and in-process monitoring. In this manner the procedures that lithographic tool users employ are simplified. In addition, correlation between imaging performance characterization and in-process monitoring is ensured. It would also be desirable for the method of forming the test structures and method of its use provide for more accurate measurements of critical dimensions (CDs) then is currently possible. Additionally, it would be desirable if these methods are applicable for essentially any size CD. Finally, it would be desirable for this method to be cost effective, both in the cost of the processing required to form the test structures and the measuring equipment used to accurately measure those test structures.